During the processing of cotton gins are used for separating the cotton fibers from the seeds. For smooth operation of the processing machinery, and in order to optimize the quality of the cotton fiber whilst minimizing output wastage, it is necessary to control the moisture of the cotton fibers during the ginning process. Specifically, cotton fibers having too high a moisture content tend to cling to the processing machinery and form wads which decompose during storage. On the other hand, cotton fibers having too low a moisture content cling to metal surfaces as a result of static electricity generated on the fibers. Furthermore, as the moisture content of the cotton fibers is lowered, their strength is reduced resulting in more frequent breakage of the fibers. As a result, the effective throughput of the processing plant is reduced.
The optimum moisture content of cotton fibers is from 6.5 to 8%. However, depending on the period during the cotton harvest, cotton may arrive at the gins with a moisture content from less than 4% to more than 12%. For these reasons, steps must be taken, during the ginning process, either to reduce the moisture content of the cotton fibers or, alternatively, to increase it in order that the resulting moisture content may fall between the stated optimum limits. A moisture measuring device is thus an integral component of the moisture control system.
Cotton bales are massive structures typically having width of 0.5 m, a height of 0.7 m and a length of 1.4 m and a weight of 250 Kg.+-.10%. Each bale typically comprises many layers which are bound together using metal bars. The moisture content through the bale may vary from one point to another by approximately 3%.
A moisture measuring device for measuring the moisture content of a cotton bale must be capable of processing one bale every 1.5-2 min for an average bale speed of approximately 1 ms.sup.-1.
U.S. Pat. No. 4,578,998 describes a moisture content measuring system for sheet material which uses microwave radiation. Two pairs of microwave radiators and receivers are combined with surface and below surface temperature measuring sensors to furnish data to a computer which interprets the data and yields moisture readings. Each pair of microwave radiator and receiver straddles the sheet test material and checks microwave transmission through the material and reflected from it, but the two radiators are crossed-polarized so that signal interchange between then is avoided.
In such a system, the two radiators are cross-polarized to each other and one irradiates the bottom surface of the test material whilst the other irradiates the top. In fact, a pair of radiators and receivers are employed: one radiator being above the sheet material and the other below, the respective receivers being aligned with the radiators and, themselves, being disposed on opposite sides of the sheet material. In order to discriminate which radiated signal emanates from which radiator and to prevent cross-talk between the two radiators, the respective microwave signals in the two radiators are cross-polarized so that each receiver receives only the signal from its corresponding radiator. However, it is to be understood that the polarization of the signals, whilst being used for signal separation, is not actually employed to determine the moisture content of the sheet material. Furthermore, the system would not appear to be suitable for measuring the moisture content of multi-layer bales of material, such as cotton, having non-uniform moisture content through a section thereof.
Additionally, when measuring the moisture content of multi-layer bales of cotton, the results will vary according to whether the layers are parallel or not and it is therefore desirable to assess the extent to which the layers are parallel or not. No provision is made in U.S. Pat. No. 4,578,998 to achieve such an objective.
It will also be noted, particularly from the Figure and accompanying description of U.S. Pat. No. 4,578,998 that the respective radiators and receivers are disposed on either side of the sheet material in very close proximity thereto. Consequently, there is very little air between the respective radiators and receivers and therefore no compensation need be provided for the ambient moisture content of the air itself. However, owing to the massive structure of cotton bales, and the like, and their non-uniformity, there inevitably is a substantial air gap between the radiators and the receivers surrounding the cotton bale. Owing to the presence of this air gap, the resulting computation of the moisture content also includes the effect of any ambient moisture in the air surrounding the bale between the respective radiator and receiver. Unless this is compensated for, the resulting computation of the bale's moisture content will be inaccurate.